Research reportFAAH inhibition attenuates TLR3-mediated hyperthermia, nociceptive- and anxiety-like behaviour in female rats
Introduction
The endocannabinoid system has been shown to exhibit potent immunomodulatory effects and represents a potential therapeutic target for peripheral and central inflammatory disorders [[1], [2], [3], [4], [5]]. N-arachidonoylethanolamine (anandamide; AEA), the most studied endocannabinoid to date, mediates its effects via cannabinoid (CB1 and CB2) and non-cannabinoid (TRPV1, PPARs and GPR55) receptors and is primarily broken down by fatty acid amide hydrolase (FAAH) [6]. in vitro and in vivo evidence have demonstrated that FAAH inhibition, and associated increases in AEA and the related N-acylethanolamines N-oleoylethanolamide (OEA) and N-palmitoylethanolamide (PEA), result in the modulation of inflammatory responses induced following the activation of the pattern recognition receptors, toll-like receptor (TLR)s [for review see [7]. (TLR)3 activation results in the induction of type 1 interferon (IFN-α and IFN-β) and NFĸB-inducible (e.g. IL-1β, IL-6 and TNF-α) inflammatory cascades which are responsible for coordinating the innate immune response to viral infection. Recent data has highlighted that FAAH inhibition attenuates the TLR3-mediated increase in the expression of IFN-inducible genes and pro-inflammatory cytokines in brain regions such as the hippocampus and hypothalamus, without altering peripheral immune responses [8,9]. The behavioural and physiological consequences of TLR3 activation include the induction of sickness behaviours such as fever/hypothermia, hypoactivity and anorexia [8,[10], [11], [12], [13]] and enhanced pain sensitivity [14] which represents a highly adaptive coping mechanism by the CNS to fight viral infection. However, aberrant activation of TLR3 can elicit adverse effects on the CNS including increased neuronal excitability and seizure susceptibility [15,16], impaired contextual and working memory [16], anxiety- and depressive-like behaviour [17] and exacerbation of underlying neurodegenerative processes [18,19]. However, it is unknown if FAAH-induced modulation of TLR3-mediated inflammatory responses result in associated physiological and behavioural changes.
Several studies have demonstrated that FAAH inhibition alters anxiety- [[20], [21], [22], [23], [24], [25]] and depressive-like behaviour [21,26] and elicits analgesic effects [22]. However, few studies have evaluated if similar effects occur in the presence of heightened inflammatory tone. The FAAH substrates AEA, OEA and PEA have been shown to modulate TLR4-induced thermoregulatory changes and hypophagia [[27], [28], [29]], most likely mediated via modulation of hypothalamic cytokine expression [28]. A recent study from our group demonstrated that FAAH inhibition modulated TLR4-mediated neuroinflammatory responses in the hippocampus and frontal cortex, an effect which was accompanied by an attenuation of TLR4-mediated anhedonia, but not sickness behaviour [30]. Furthermore, FAAH inhibition has been demonstrated to reverse TLR4-mediated mechanical allodynia [31], thermal hyperalgesia and paw oedema [32]. Collectively, these results demonstrate a role for FAAH substrates in the modulation of behavioural responses following TLR4 activation, although there are no studies to date examining if similar responses occur following activation of other TLRs such as TLR3. Thus, the aim of the present study was to examine the effect of enhancing FAAH substrate levels on TLR3-mediated neuroimmune activation and resulting physiological and behavioural responses.
Section snippets
Animals
Experiments were carried out on female Sprague-Dawley rats (weight, 180–250 g; NUI Galway breeding facility), housed singly for at least 3 days prior to the experiment in transparent plastic bottomed cages (48 cm × 20 cm × 27 cm) containing wood shavings as bedding. The animals were maintained at a constant temperature (21 ± 2 °C) under standard light-dark cycle conditions (12: 12 h light-dark, lights on from 0700 to 1900 h). All experiments were carried out during the light phase between
FAAH inhibition attenuates TLR3-mediated fever but not sickness behaviour
In order to evaluate if enhancing FAAH substrate levels modulates TLR3-mediated acute physiological and sickness responses, the effect of systemic URB597 administration on body temperature, locomotor activity and body weight over a 24 h period following TLR3 activation using the viral mimetic poly I:C was examined. There was no significant difference in temperature at baseline between groups [one-way ANOVA; p > 0.05 (vehicle-saline 36.7 °C ± 0.3; vehicle-poly I:C 36.4 °C ± 0.4; URB597-poly I:C
Discussion
Aberrant TLR activation may underlie a host of psychiatric and neurodegenerative disorders and thus greater understanding of TLR-associated physiological and pathophysiological responses is of significant fundamental and therapeutic importance. The results of the present study demonstrate that the systemic administration of the FAAH inhibitor URB597 attenuated TLR3-mediated microglia/macrophage activation and some, but not all, associated behavioural changes in female rats. Essentially, TLR3
Author’s roles
LF, MR and DF were involved in the experimental design and writing the manuscript. LF, DK and MR were involved in conducting and analysing the experiments. All authors have read and approved the manuscript
Declaration of Interest
None.
Acknowledgements
The authors would like to thank Edel Hughes, Marykate Killilea and Rachel Humphreys for their assistance with experimental procedures.
The authors would like to gratefully acknowledge funding received from the Hardiman Postgraduate Research Scholarship, The discipline of Physiology and The College of Medicine, Nursing and Health Sciences, National University of Ireland Galway. The funding sources had no input into the study design, collection, analysis and interpretation of the data or writing
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